P.E. Vickers et al. Carbon 38(2000)675-689 edure. In addition to these observations, it was also noted acid-base properties by in surace as chromatography by Proctor and Sherwood [6, 7 that the plasmon satellite (IGC). It was found that a su eatment of 24 Cg feature observed at a 6-7 ev higher binding energy than increase of the surface by the primary peak decreased in intensity as the oxidation of nearly five-fold. Curve-fitting of the C Is spectrum led to the graphite fibre surface progressed. This was probably a the conclusion that the C-o/C-N moiety increased fro result of the disruption of t-bonding orbitals at the fibre 0% to 4%, C=0(carbonyl)from 0% to 9%, and o-C-Oh surface caused by the introduction of oxygen function- (carboxyl) from 2% to 8% of the total carbon present in alities. Nakao et al. [12 also oxidised ex-PAN fibres the surface. This result was also supported by titrimetric Pyrofil XTR40) in both ammonium bicarbonate and analysis. IGC results showed an increase in acidity and phosphoric acid electrolytes and concluded that the equiva- basicity, attributed to the introduction of alcohol and lent phosphoric acid treatment introduced a greater propor- carboxylic acid groups (amphoteric)and carbonyl groups tion of oxygen to the fibre surface, in agreement with(basic). Somewhat surprising was the observation that y Sherwood et al. It was also found that the ILSS of a CFrP increased significantly as the oxidation treatment pro- increased for fibres that had received both types of gressed. At a treatment level of 24 cg, the fibres electrochemical oxidation, but treatment in the phosphori possessed a y. of 140 mJ m, a value greater than acid electrolyte was detrimental to the transverse flexural previously obtained for highly ordered pyrolytic graphite strength of the composite. This was attributed to the(hoPg) by IGc [14 of 129 mJ m. As the untreated formation of fissures on the fibre surface causing a weak fibres are highly graphitic in nature, y s should be expected to decrease as the oxidation progresses, because of disrup- It is important to relate the chemical composition of the tion to the bonding in the aromatic ring structure. Nardin et fibre surface to the surface free energy so that a measure of al. [15 used IGC to measure the surface free energy of the wettability of the fibre surface may be obtained. The Soficar T300 fibres. These fibres are similar to AKzo surface free energy of a solid, y, may be thought of as the HTA 5000 fibres in both surface chemical and physical sum of the dispersive component to the surface free properties. It was found that y s was virtually unaffected energy, y, and the polar or Lewis acid-base component (y:=50 m m by the oxidation treatment employed and to the surface free energy, y P In order for spontaneous that both the acidity and basicity increased as a result of etting by a liquid to occur, y must exceed the surface oxidation. The change in surface free energy monitored by tension,y, of the liquid in question. It therefore follows, DCAA has also been correlated with data from XPS that. in order for complete wetting of the carbon fibl experiments by Wu et al. [16] for Hercules As(oxidised) surface by a resin to occur, it would be advantageous to and aU (unoxidised) HS fibres. It was found that y produce a fibre with a high surface energy. However, it is decreased slightly as a result of the standard commercial well known that graphite has a much higher surface energy treatment and this was accompanied by a very large (120 mJ m )than water (72. 8 mJ m ) but is not wet increase in yP, as might be expected, leading to an at all well by water. Therefore, instead of the comparison increase in y of slightly less than a factor of two. It was of the surface free energies of a solid and a liquid, it is also found that heating the fibres in a vacuum to tempera more accurate to relate wettability to the surface free tures as high as 1000C led to a significant decrease in y energy of the interface, y. The latter must be minimised presumably because of degradation of the polar surface in order to improve wettability, for example, via strong groups introduced as a result of electrochemical oxidation. acid-base interactions at the solid-liquid (i.e. fibre-resin) In this paper we compare acid-base properties of fibres interface. This is achieved by introducing polar oxygen (AKZO's Tenax hta high-strength fibres) that have containing functionalities which may react chemically to received varying degrees of a manufacturers standard form a covalent bond at the fibre surface. The introduction eectrochemical oxidation, measured by both IGC and of polar groups to the surface will enhance the probability DCAA, with the elemental composition of the fibre surface of a Lewis acid-base interaction between the two. Bradley measured by XPs. IGC brings thermodynamic information [13] compared XPS and dynamic contact angle analysis at the molecular scale whereas dCaa probes the wettabili- (DCAA)results of oxidised and unoxidised fibres(akzo ty of material surfaces at the macroscopic scale. We thus Tenax HTA), and found that the oxygen content of the expect complementary information to be obtained from the fibre surface was increased by a factor of 2. Surprisingly, two methods. Another technique that may also be used to the DCAA results indicate that y i decreases after the elucidate chemical structure of the fibre surface is time-of- oxidation treatment. This effect was attributed to a thin flight secondary ion mass spectrometry (ToF-SIMS).Al- oxygen-rich layer thought to be present on the surface of though this is a more recent development than XPS, it has untreated fibres that is removed during the initial stages of rarely been used for the characterisation of unmodified anodic oxidation. Mahy et al. b3 electrochemically oxi- carbon fibre surfaces. This is because sample preparation is dined AKZO Tenax HTA 5000 fibres in an ammonium not trivial as a result of field emission occurring from stray sulphate electrolyte, and analysed the chemical composi- fibre ends. In addition to this, the highly aromatic nature of tion of the fibre surface by XPS and investigated the the fibre surface leads to relatively simple spectra of676 P.E. Vickers et al. / Carbon 38 (2000) 675 –689 cedure. In addition to these observations, it was also noted acid–base properties by inverse gas chromatography21 by Proctor and Sherwood [6,7] that the plasmon satellite (IGC). It was found that a surface treatment of 24 C g feature observed at a 6–7 eV higher binding energy than increased the oxygen concentration of the surface by the primary peak decreased in intensity as the oxidation of nearly five-fold. Curve-fitting of the C 1s spectrum led to the graphite fibre surface progressed. This was probably a the conclusion that the C–O/C–N moiety increased from result of the disruption of p-bonding orbitals at the fibre 0% to 4%, C=O (carbonyl) from 0% to 9%, and O=C–OH surface caused by the introduction of oxygen function- (carboxyl) from 2% to 8% of the total carbon present in alities. Nakao et al. [12] also oxidised ex-PAN fibres the surface. This result was also supported by titrimetric (Pyrofil XTR40) in both ammonium bicarbonate and analysis. IGC results showed an increase in acidity and phosphoric acid electrolytes and concluded that the equiva- basicity, attributed to the introduction of alcohol and lent phosphoric acid treatment introduced a greater propor- carboxylic acid groups (amphoteric) and carbonyl groupsd tion of oxygen to the fibre surface, in agreement with (basic). Somewhat surprising was the observation that g s Sherwood et al. It was also found that the ILSS of a CFRP increased significantly as the oxidation treatment pro- 21 increased for fibres that had received both types of gressed. At a treatment level of 24 C g , the fibres d 22 electrochemical oxidation, but treatment in the phosphoric possessed a g s of 140 mJ m , a value greater than acid electrolyte was detrimental to the transverse flexural previously obtained for highly ordered pyrolytic graphite 22 strength of the composite. This was attributed to the (HOPG) by IGC [14] of 129 mJ m . As the untreated d formation of fissures on the fibre surface causing a weak fibres are highly graphitic in nature, g s should be expected boundary layer. to decrease as the oxidation progresses, because of disrup￾It is important to relate the chemical composition of the tion to the bonding in the aromatic ring structure. Nardin et fibre surface to the surface free energy so that a measure of al. [15] used IGC to measure the surface free energy of the wettability of the fibre surface may be obtained. The Soficar T300 fibres. These fibres are similar to AKZO surface free energy of a solid, g , may be thought of as the HTA 5000 fibres in both surface chemical and physical s d sum of the dispersive component to the surface free properties. It was found that g s was virtually unaffected d d 22 energy, g s, and the polar or Lewis acid–base component (g s¯50 mJ m ) by the oxidation treatment employed and p to the surface free energy, g . In order for spontaneous that both the acidity and basicity increased as a result of s wetting by a liquid to occur, gs must exceed the surface oxidation. The change in surface free energy monitored by tension, gl , of the liquid in question. It therefore follows, DCAA has also been correlated with data from XPS that, in order for complete wetting of the carbon fibre experiments by Wu et al. [16] for Hercules AS (oxidised)d surface by a resin to occur, it would be advantageous to and AU (unoxidised) HS fibres. It was found that g s produce a fibre with a high surface energy. However, it is decreased slightly as a result of the standard commercial well known that graphite has a much higher surface energy treatment and this was accompanied by a very large 22 22 p (.120 mJ m ) than water (72.8 mJ m ) but is not wet increase in g s, as might be expected, leading to an at all well by water. Therefore, instead of the comparison increase in g of slightly less than a factor of two. It was s of the surface free energies of a solid and a liquid, it is also found that heating the fibres in a vacuum to tempera-p more accurate to relate wettability to the surface free tures as high as 10008C led to a significant decrease in g , s energy of the interface, g . The latter must be minimised presumably because of degradation of the polar surface sl in order to improve wettability, for example, via strong groups introduced as a result of electrochemical oxidation. acid–base interactions at the solid–liquid (i.e. fibre–resin) In this paper we compare acid–base properties of fibres interface. This is achieved by introducing polar oxygen (AKZO’s Tenax HTA high-strength fibres) that have containing functionalities which may react chemically to received varying degrees of a manufacturers standard form a covalent bond at the fibre surface. The introduction electrochemical oxidation, measured by both IGC and of polar groups to the surface will enhance the probability DCAA, with the elemental composition of the fibre surface of a Lewis acid–base interaction between the two. Bradley measured by XPS. IGC brings thermodynamic information [13] compared XPS and dynamic contact angle analysis at the molecular scale whereas DCAA probes the wettabili- (DCAA) results of oxidised and unoxidised fibres (AKZO ty of material surfaces at the macroscopic scale. We thus Tenax HTA), and found that the oxygen content of the expect complementary information to be obtained from the fibre surface was increased by a factor of 2. Surprisingly, two methods. Another technique that may also be used to p the DCAA results indicate that g decreases after the elucidate chemical structure of the fibre surface is time-of- s oxidation treatment. This effect was attributed to a thin flight secondary ion mass spectrometry (ToF-SIMS). Al￾oxygen-rich layer thought to be present on the surface of though this is a more recent development than XPS, it has untreated fibres that is removed during the initial stages of rarely been used for the characterisation of unmodified anodic oxidation. Mahy et al. [3] electrochemically oxi- carbon fibre surfaces. This is because sample preparation is dised AKZO Tenax HTA 5000 fibres in an ammonium not trivial as a result of field emission occurring from stray sulphate electrolyte, and analysed the chemical composi- fibre ends. In addition to this, the highly aromatic nature of tion of the fibre surface by XPS and investigated the the fibre surface leads to relatively simple spectra of